Weathering (AQA A-Level Geography): Revision Notes

Weathering

Introduction to sub-aerial weathering

Coastal landscapes are continuously shaped by various processes, with weathering playing a crucial role. At the coast, there are two main types of processes that shape the environment: those occurring in the sea (marine processes) and those happening on land (sub-aerial processes). This note focuses on sub-aerial weathering - the land-based processes that gradually break down coastal rocks and cliffs.

Sub-aerial weathering is particularly important at the coast because of the constant presence of air, moisture, and cycles of wetting and drying. The processes can be grouped into three main categories:

• Mechanical (or physical) weathering
• Biological weathering
• Chemical weathering

Each type of weathering works in different ways, but they often work together to break down coastal rocks more effectively than any single process could alone.

Mechanical weathering

Mechanical weathering, also called physical weathering, involves the physical breakdown of rocks without changing their chemical composition. The rock is broken into smaller pieces, but the rock type itself remains the same. These processes are strongly influenced by climate conditions, particularly temperature and moisture availability.

Freeze-thaw weathering

This is one of the most effective weathering processes in areas where temperatures regularly fluctuate above and below freezing point.

Freeze-thaw is particularly common in areas with ready water supply and where temperatures fluctuate around freezing point. At the coast, this might occur on cliffs that are regularly wetted by sea spray. The process is most effective where there is a rocky supply of water and frequent temperature changes across the freezing point.

Pressure release

Pressure release, sometimes called dilation, occurs through a different mechanism:

• Over time, erosion, weathering, and mass movement gradually remove material from above a rock layer
• This removal of overlying material (which could be hundreds of metres thick) reduces the pressure on the rocks beneath
• The rock beneath had been compressed by the weight above it
• When this weight is removed, the rock is able to expand slightly
• This expansion causes the development of cracks, joints, and weaknesses in the rock structure
• These new weaknesses make the rock much more susceptible to other weathering and erosion processes

Biological weathering

Biological weathering describes the breakdown of rocks through the actions of living organisms. At the coast, this is an active and important set of processes because of the abundance of plant and animal life in coastal environments.

Marine organisms

Many coastal organisms have evolved special adaptations that allow them to bore into solid rock:

• Certain shellfish, such as the piddock, have specially adapted shells that function as drilling tools
• These organisms physically bore into the rock to create holes where they can live
• This activity is particularly common in areas with chalk geology, creating a distinctive sponge-like, pitted rock surface
• The holes and cavities weaken the rock structure significantly

Seaweed and other attached organisms

Plant life also contributes to weathering:

• Seaweed attaches itself firmly to rocks using strong holdfasts
• Wave action pulls on the seaweed, which in turn pulls on the rock
• This repeated pulling force can be sufficient to prise away loose fragments of rock from the seafloor or cliff base
• Over time, this removes material and exposes fresh rock to further weathering

Chemical secretion by organisms

Some organisms actively produce chemicals that break down rock:

• Certain types of algae and other small organisms secrete chemicals that promote the solution of rock minerals
• These chemicals speed up the chemical weathering of the rock surface
• This process works alongside purely chemical weathering to break down the rock more rapidly

Animal activities

Larger animals also contribute to coastal weathering:

• Birds such as rabbits and cliff-nesting seabirds burrow into or dig into cliffs
• This burrowing activity physically breaks up the rock and soil
• It also creates pathways for water to enter, which then enables other weathering processes (particularly freeze-thaw and chemical weathering) to work more effectively
• The disturbance of the rock structure makes it more vulnerable to erosion

Chemical weathering

Chemical weathering occurs when rocks are exposed to air and moisture, causing chemical reactions that break down the rock minerals. This is often the most significant type of weathering at the coast because of the constantly moist conditions and the presence of salt water. There are several important chemical weathering processes:

Solution

Solution is one of the main chemical weathering processes operating at the coast.

• This process involves rock minerals dissolving directly in water
• It is particularly effective on rocks like limestone and chalk, which contain calcium carbonate
• Solution combines with erosion processes to create many distinctive coastal features
• Salt water can be more effective than fresh water for solution because it contains dissolved chemicals that enhance the dissolving process

Oxidation

Oxidation is a chemical reaction between oxygen and rock minerals:

• Oxygen dissolved in water reacts with certain minerals in the rock
• This reaction forms new compounds called oxides and hydroxides
• These new compounds are often weaker than the original minerals
• The process is particularly effective on ferrous (iron-rich) rocks
• Oxidation is visible as a brownish or yellowish staining on the rock surface
• This discolouration indicates that the rock structure has been weakened
• The weakened rock becomes more susceptible to other forms of weathering and erosion

Iron-rich rocks are especially vulnerable to oxidation, which is why you often see rust-coloured staining on coastal cliffs containing iron minerals.

Hydration

Hydration involves the physical addition of water molecules to minerals within the rock:

• Water becomes incorporated into the crystal structure of certain minerals
• This causes the minerals to expand in volume
• The expansion creates stress within the rock
• These stresses can cause cracks to form or existing cracks to widen
• The process weakens the overall rock structure
• Once weakened, the rock becomes more vulnerable to other weathering processes

Hydrolysis

Hydrolysis is a more complex chemical reaction:

• Mildly acidic water reacts with minerals in the rock
• This reaction produces new minerals in the form of salts
• The salts that form are typically weaker than the original parent rock
• Both the chemical reaction itself and the weaker products contribute to rock breakdown
• The rock becomes increasingly susceptible to further degradation

This process is particularly effective on rocks containing feldspar minerals, which are common in many rock types.

Carbonation

Carbonation is an especially important process for breaking down limestone and chalk coastlines:

Carbonation is responsible for many distinctive features in limestone coastal areas, working alongside marine erosion processes.

Acid rain

Acid rain represents an enhanced form of natural chemical weathering:

• Even without pollution, rainwater is slightly acidic due to naturally occurring carbon dioxide in the atmosphere
• However, industrial activity and the burning of fossil fuels release additional gases into the atmosphere
• Sulphur dioxide and nitric oxides from these sources react with rainwater
• This creates rainwater with weak sulphuric and nitric acids
• This more acidic rain (acid rain) reacts with various minerals in different rock types
• The acids can weaken or even dissolve these minerals
• This accelerates the natural weathering process

Remember!